It has been widely acknowledged that carbon dioxide (CO2) concentration has a strong correlation with global warming that increases the chances of natural disasters such as hurricane and flooding. Power plants contribute approximately one third of all CO2 emissions related to human activities, with each power plant capable of emitting a million tons of CO2. In addition, other industries, such as oil refineries, cement works, and iron and steel production processes, also emit large amounts of CO2 from each plant. These emissions could be reduced substantially, without major changes to the basic process, by capturing and storing the CO2.
CO2 emissions can be reduced in many ways, such as increasing the efficiency of power plants or by switching coal to natural gas. However, these methods alone will not achieve the desirable CO2 emission level. Since a significant amount of CO2 emission comes from fossil fuel combustion, sequestration of CO2 from fossil fuel combustion could play an important role in control of total CO2 emission levels. Sequestration of CO2 refers to the technologies of capturing CO2 emitted from a source and then securely storing the captured CO2 that can stay for hundreds or even thousands of years. Reservoirs under the earths surface and in the oceans such as depleted oil and gas reservoirs, deep saline aquifers, and unminable coal seams are the common places used for CO2 storage today.
In terms of CO2 capture generated from combustion of a primary fossil fuel, there are three main approaches: post-combustion, pre-combustion and oxyfuel combustion. Pre-combustion CO2 capture is used for fossil fuel gasification that coverts the fuel to carbon monoxide and hydrogen first, and then, the carbon monoxide reacts with steam in a second reactor (a “shift reactor”) to generate additional hydrogen and CO2. Finally, the CO2 gas is separated from hydrogen that is a carbon-free energy carrier and can be combusted to generate power and/or heat. The pre-combustion method would be suitable for power plants employing integrated gasification combined cycle (IGCC) technology.
The oxyfuel combustion method does not use the air but uses oxygen separated from the air as the fuel combustion oxidant. The oxygen rich combustion results in high CO2 concentration (greater than 80% by volume) and water vapor. Water vapor can be condensed by cooling and compressing the gas stream, and what is left is mainly CO2. The requirement of oxygen separation from the air reduces the energy efficiency for oxyfuel combustion system. In addition, the oxyfuel combustion system also needs to remove air pollutants and non-condensed gases (such as nitrogen) from the flue gas before CO2 is sent to storage. Boiler systems using oxyfuel combustion are in the demonstration phase.
An advantage of the post-combustion CO2 capture is that it is easy to retrofit the CO2 capture system without major changes of the boiler designs. For a modern coal-fired power plant or a natural gas combined cycle (NGCC) power plant, a liquid solvent CO2 capture system is commonly used as the post-combustion CO2 capture solution, which typically uses an organic amine such as monoethanolamine (MEA) to capture CO2 (typically 3-15% by volume in a flue gas stream). The system is normally installed downstream of the SO2 scrubber. Since CO2 is an acidic gas or a weak acid, it is adsorbed via the reaction with MEA. This weak acid-weak base reaction is reversible, and CO2 and MEA can be recovered upon heating. A typical system consists of an “absorber” where CO2 is captured and a “stripper” where CO2 is separated from MEA. Post-combustion systems can remove up to 95% of CO2. MEA-based post-combustion CO2 capture systems are commercially available but the system operation consumes a significant amount of energy. Other disadvantages of MEA-based systems are the corrosive and volatile natures of the organic base.
The inventors have now developed a new sorbent structure that may be used, for example, to remove CO2 from a combustion flue gas stream of a coal-fired power plant or a NGCC power plant. Such a structure may provide, for example, low pressure drop, high surface area, and excellent energy efficiency for post-combustion CO2 capture.